This research is directed at providing an ultrastructural description of physiologically-defined circuitry in the lateral geniculate nucleus of cats. Emphasis is placed on a description of retinogeniculate circuitry and differences among the W-, X-, and Y-cell pathways. Such information is not only fundamental to an understanding of the neural basis of vision but also can serve as a framework against which to compare neural correlates of certain forms of amblyopia. Structure and function are related at the single cell level in the following manner. Physiological recordings of a single axon or cell are accomplished intracellularly with a fine micropipette filled with horseradish peroxidase (HRP), and HRP is then iontophoresed into the physiologically defined neuronal element. The HRP completely fills a soma and its dendrites or an axon and its terminal boutons, and the HRP reaction product is used as an electrondense marker for subsequent ultrastructural analysis. Electron microscopic reconstructions from such material permit a fairly complete description of synaptic circuits related to single, physiologically identified cells. Plans are described to analyze both the geniculate terminals of various afferents (W-, X-, or Y-cell optic tract, perigeniculate, corticogeniculate) as well as the synapses formed onto geniculate W-, X-, and Y-cells.